The invention relates generally to DC feeder circuit breaker control systems. More particularly, the invention relates to methods and apparatus for automatic reclosing of a circuit breaker.
DC circuit breakers are well known. One particular application is the use of a circuit breaker in a DC feeder line for a transit system. The DC power for electrically operated trains, for example, is provided from one or more electric substations through a DC feeder line and return. A circuit breaker is connected in the feeder line circuit so that when a fault occurs the breaker is tripped and power is disconnected from the feeder line and track circuit. The circuit breaker is not permitted to be reclosed until it is first determined that the fault no longer exists in the feeder line and track circuit.
Over the years, different control systems have been designed to provide automatic reclosure of the breaker after a trip occurs. Typically, an attempt to reclose a breaker is made after first checking the feeder line for a fault condition. This is done by performing a check generally known as a load measurement test. A load measurement test typically is performed by injecting a test current into the feeder circuit, such as by connecting a load measurement resistor (LMR) between the DC main power bus (connected to the substation supply) and the feeder line. The feeder line voltage referenced to the return line can then be used to determine the load on the feeder circuit and hence the presence or absence of a fault condition. Typically, a fault condition corresponds to a feeder circuit impedance of 0.2 to 1 ohm or less.
Known systems for automatic reclosing of a feeder circuit breaker typically use a combination of cam timers, latching relays, meter relays and timers. Although such apparatus can be used successfully, their designs have inherent limitations and aspects that are not ideal. Latching relays are expensive, and cam timers are difficult to adjust to required timing accuracies. Meter relays tend to be sensitive to induced transient voltages in the associated switchgear, such as by breaker closure and relay operations. The different types of control devices used in such known systems also tend to require different power sources, rather than all being powered from a reliable source such as the station battery. Overall, the systems tend to be quite complex and expensive without an appreciable degree of flexibility. Each system is typically designed as a unique system for a particular application.
It is desirable, therefore, to provide an automatic reclosing system that uses more state of the art technology, such as programmable solid state controllers. However, efforts to date have proven to be less than adequate because such systems fail to accurately account for negative return voltage, and also the fact that such systems are susceptible to drift from temperature variations. Calibration of such systems is typically based on adjustments at the factory, not in the field. This is because known programmable controller systems use transducer devices that provide for transformer coupled isolation of the high voltage feeder circuit from the controller hardware. Such isolation is used in large measure as a safety consideration to isolate the track potentials from the control circuit. Once the hardware is installed in a switchgear, however, there is no way to calibrate the system because the controller simply receives the transducer output as a control parameter. The use of transformer coupled isolated transducers to the extent known heretofore has also not permitted the inclusion of a self-test capability of the automatic reclosing system.
A significant problem with known automatic reclosing systems is that they tend to be sensitive to the presence of negative return voltage, such as occurs with multiple track and feeder circuits. This negative return voltage affects the accuracy of the load measurement process.
The objectives exist, therefore, for an automatic reclosing system for a DC feeder circuit breaker, which system can be easily configured for different installations, while also being capable of self-test functions, calibration, and offset error correction. Such a system should also include flexibility when configuring the system for a particular installation.